1. Field of the Invention
This invention relates to wireless communications and, more particularly, to a transport channel multiplexing system that supports a downlink shared channel system.
2. Description of Related Art
Wireless communications systems include conventional cellular communication systems which comprise a number of cell sites or base stations, geographically distributed to support transmission and receipt of communication signals to and from wireless units which may actually be stationary or fixed. Each cell site handles voice communications over a particular region called a cell, and the overall coverage area for the cellular communication system is defined by the union of cells for all of the cell sites, where the coverage areas for nearby cell sites overlap to some degree to ensure (if possible) contiguous communications coverage within the outer boundaries of the system's coverage area.
When active, a wireless unit receives signals from at least one base station or cell site over a forward link or downlink and transmits signals to (at least) one cell site or base station over a reverse link or uplink. There are many different schemes for defining wireless links or channels for a cellular communication system, including TDMA (time-division multiple access), FDMA (frequency-division multiple access), and CDMA (code-division multiple access) schemes. In CDMA communications, different wireless channels are distinguished by different codes or sequences that are used to spread different information streams, which may then be modulated at one or more different carrier frequencies for simultaneous transmission. A receiver can recover a particular information stream from a received signal using the appropriate code or sequence to decode the received signal.
Due to the delay-intolerant nature of voice communication, wireless units in conventional cellular systems transmit and receive over dedicated links between a wireless unit and a base station. Generally, each active wireless unit requires the assignment of a dedicated link on the forward link and a dedicated link on the reverse link. Traditional data applications are typically bursty and, unlike voice communications, relatively delay tolerant. As such, using dedicated links to transmit data is an inefficient use of network resources. Wireless communications systems are evolving that will support a variety of data services, such as wireless web browsing.
In the Universal Mobile Telecommunications System (UMTS), wireless units communicate with a base station over dedicated channels. To provide efficient wireless data communications, UMTS uses a downlink shared channel which can be shared by a plurality of wireless units to receive data. The downlink shared channel (DSCH) structure accepts information from a number of transport channels, that in general address different users, time multiplexes/schedules those into a single Coded Shared (or Scheduled) Transport Channel (CSTrCH). The CSTrCH is then mapped into a number of Physical Channels each corresponding to an Orthogonal Variable Spreading Factor (OVSF) code. To more efficiently use the downlink to carry data, it is advantageous to provide a transport channel multiplexing structure which enables more efficient use of wireless resources on the downlink.